X-ray imaging benefits from the ability of X-rays to penetrate through materials. Organic materials, such tumours, explosives or archeological artefacts, are largely transparent to high energy x-rays (wavelength >1 nm), and are therefore hard to image using traditional absorption contrast. Although high energy X-rays are weakly absorbed by materials, they are refracted, and this can be used to produce a phase-contrast image of objects. Phase-contrast image provides additional information on the internal structure of the object, and in case of organic materials this information can be more detailed than the absorption contrast. Additionally, interfaces and irregularities inside the imaged object scatter the x-rays, such that dark-field contrast carries even more information about the studied specimen. This additionally available phase- and dark-field contrast information can in some cases be much more detailed than in the case of the traditional absorption-contrast imaging, and can significantly improve diagnostics decision making, enhance failure analysis in material science, improve safety in airports by improved detection of forbidden organic substances (explosives, narcotics), benefit the geological studies (e.g., oil and diamond imaging), and wood/paper industry.
Grating-based interferometry is an established method for producing phase-contrast and dark-field contrast images. With reference to FIGS. 1a and 1b, in the method, a coherent beam 11 of light impinges on a diffraction grating G1 and an intereference pattern 13A is formed behind the grating. At certain distances close to the grating, the intereference pattern resembles periodic structure of high and low intensities. If an organic object 10 (for instance tissue with a tumour) is positioned in the beam, the additional interaction of the beam 11 with the object 10 produces a modified beam 12, distorts the original periodic interference pattern 13A and produces a modified interference pattern 13B. A detector 15 can detect the distortions and the object's 10 image can be reconstructed. Usually the useful, object-related signals are weak and the image contrast is blurred by a large background. The useful signal is differentiated from the background by positioning an absorption grating G2 in front of the detector 15. This grating G2 completely absorbs the periodic intensities in the absence of the imaged object (see FIG. 1a). When the object is positioned in the beam 11, only the distortions due to the object are able to reach the detector 15 (see FIG. 1b).
Phase contrast microscopy allows imaging of transparent objects that are otherwise invisible in absorption contrast. Small phase variations are introduced into the propagating light, for example due to different refractive indices of neighbouring regions in the specimen, or due to their different densities. These small phase differences are translated into the variations of light intensity to visualize the transparent structures that would otherwise be invisible in absorption contrast. The access to the phase information can be obtained through a number of techniques. For the X-ray radiation, the most promising method is grating-based interferometry. In this method, a coherent X-ray beam impinges on a grating. The grating splits the beam into several components, that interefere with each other and produce periodic patterns in the near-field region. For a phase grating, i.e. (nearly) transparent grating that introduces a phase-shift into the incoming beam, with a phase-shift of π rad, the periodic patterns occur at certain distances
            d      ⁡              (        N        )              =          N      ⁢              1        8            ⁢                        p          2                λ              ,      N    =    1    ,  3  ,      5    ⁢                  ⁢    …  
For an absorption grating or a phase grating with a phase shift of π/2, the periodic patterns occur at distances
            d      ⁡              (        N        )              =          N      ⁢              1        8            ⁢                        p          2                λ              ,      N    =    4    ,      8    ⁢                  ⁢    …  where p and λ are the periodicity of the grating and the wavelength, respectively. A phase object introduces additional variations into the primary beam such that the periodic near-field intereference pattern is distorted. An absorption grating is aligned in front of the detector, such that in the absence of an object, the grating absorbs the periodic pattern and no signal is detected. Due to the phase object, the interference fringes are shifted or distorted, such that some intensity is able to bypass the absorption grating and reach the detector.
To separate the object related phase shift information from other contributions, e.g. due to the imperfections of the illumination and the phase grating, the phase-stepping approach is used. One of the gratings, usually the absorption grating G2, is scanned over one period of the grating and for every point of the scan an image is taken. Usually, the similar procedure is repeated without the object to provide the background or reference signal to be subtracted from the images obtained with the object in the beam. For a sinusoidally varying signal on every pixel versus the grating shift, the minimum number of images that needs to be taken is three, or six images—if the reference images are taken too. However, to ensure a good quality of the image usually more images need to be taken.
Practically all currently available grating-based differential phase contrast methods rely on the use of absorption gratings to reduce the background and improve the image contrast. The grating is very expensive because it needs to be made of Au or other heavy material, needs to have a very fine pitch and be very tall. Further, the absorption grating limits the X-ray energy that can be used. Higher X-ray energies are beneficial for imaging of very thick specimens and help to reduce the radiation dose, e.g., in diagnostics imaging. Moreover, the signals are relatively low and the detector needs to integrate the intensities over long times. However, the image contrast of the resulting image is typically relatively low.
Grating-based differential phase-contrast imaging is discussed in more detail e.g. in Weitkamp et al, “X-ray phase imaging with a grating interferometer”, Optics Express, Vol. 12, No. 16, 8 Aug. 2005 and Pfeiffer et al, “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray sources”, nature physics, Vol. 2, April 2006.
There are also a plurality of variations of the basic method, including variations to the grating configurations. Some of them are discussed in US 2011/0013743, US 2012/0057676 and US 2010/0246764. The last one of these describes a simplified fabrication method of an absorption source grating placed between the x-ray source and the object to be imaged, to improve the coherence of non-coherent X-ray sources. In addition, WO 2011/157749 discloses an inclining phase grating placed between the object and detector and having the advantage that it is suitable for different setups, energies, distances etc. without a need of making new gratings for every new setup. WO 2011/096584, on the other hand, discloses a “phase stepping” method where information about the shifts of interference fringes due to the refraction in the object is obtained by inclining or rotation of a grating in front of the detector.
The proposed variations still necessitate the use of a highly absorbing grating in front of the detector and therefore suffer from at least some of the same problems as the basic method. In addition, the contrast of the resulting images is not optimal, since the background image resulting from rays penetrating the absorption grating is still strong. Moreover, signal to noise ratios could be better. Also radiation doses are relatively high due to the low x-ray energies that need to be used.
In addition, the method does not provide a readily obtained phase contrast image. The object-phase-related image needs to be reconstructed (numerically) from a plurality of digital images.
Thus, there is a need for improved x-ray imaging methods and apparatuses.